Flow control system

Abstract: When actuators (5a,5b) are driven in parallel, load pressures from load lines (4a,4b) are guided to opening drive units (16a,16b) of pressure compensation valves (9a,9b) whereas upstream-side pressures of correcting variable restrictors (10a,10b) are guided to closing drive units (14a,14b). When downstream-side pressures of operating variable restrictors (11a,11b) exceed the sums of the load pressures and the spring forces of springs (15a,15b), closing drive forces are increased so that the openings of the valves (9a,9b) are reduced. When load pressures increase or the upstream-side pressures of the restrictors (11a,11b) drop, drive forces applied in the opening directions to the valves (9a,9b) increase relative to the upstream-side pressures so lowered so that the openings of the valves (9a,9b) are increased. The valves (9a,9b) are hence automatically controlled to keep constant the pressure differences across the restrictors (10a,11a; 10b,10b), whereby the drive speeds of the actuators (5a,5b) are maintained constant irrespective of the load pressures as long as the corresponding restricted openings remain constant.

Description: The present invention relates to a flow control system arranged in a hydraulic circuit provided with a variable displacement hydraulic pump, plural actuators driven by pressure oil fed from said hydraulic pump and pump control means for controlling the displacement of said hydraulic pump so that a delivery pressure of said hydraulic pump becomes higher by a predetermined value than a maximum load pressure to said plural actuators, whereby the flow rate of the pressure oil to be fed to said actuators is controlled, the flow control system further comprising plural valve means connected between said hydraulic pump and said actuators, respectively, and pressure compensation valves arranged on upstream sides as viewed in pressure oil feeding directions of said plural valve means disposed corresponding to said actuators. Such a flow control system is known from EP-A-0 440 802. In this document the load sensing system is provided with a "before orifice" type pressure compensation valves for maintaining a constant differential pressure with which each actuator can be driven. The term "before orifice" means that a pressure compensation valve is arranged on an upstream side of the valve means. The technique disclosed in said document is only concerned with load sensing control of a pump. It is not possible to adjust flow distribution among individual actuators. The purpose of the invention is to overcome this disadvantage. Therefore, according to the invention, the flow control system as described in the opening paragraph, is characterized in that each of said plural valve means is provided with a restrictor group with an operating variable restrictor, whose opening can be varied by control means to control the flow rate of the pressure oil to be fed to the corresponding actuator, and a correcting variable restrictor for correcting a pressure difference across said operating variable restrictor, the opening of each of said pressure compensation valves being independently set by a drive force in an opening direction based on a downstream pressure of the corresponding operating variable restrictor, a drive force in the opening direction set by pressure-difference setting means and a drive force in a closing direction based on an upstream pressure of the corresponding, restrictor group so that the upstream pressure of the corresponding restrictor group becomes higher by a predetermined pressure than the downstream pressure of the corresponding restrictor group. The present invention further relates to a flow control system arranged in a hydraulic circuit provided with a variable displacement hydraulic pump, plural actuators driven by pressure oil fed from said hydraulic pump and pump control means for controlling the displacement of said hydraulic pump so that a delivery pressure of said hydraulic pump becomes higher by a predetermined value than a maximum load pressure of said plural actuators, whereby the flow rate of the pressure oil to be fed to said actuators is controlled, the flow control system further comprising plural valve means connected between said hydraulic pump and said actuators, respectively, each of said plural valve means having an operating variable restrictor, whose opening can be varied by control means to control the flow rate of the pressure oil to be fed to the corresponding actuator, and pressure compensation valves arranged on downstream sides as viewed in pressure oil feeding directions of said operating variable restrictors disposed corresponding to said actuators. Such a flow control system is known from EP-A-0439621. In this document the load sensing system is provided with an after orifice type pressure compensation valves. The term "after orifice" means that a pressure compensation valve is arranged on a downstream side of the valve means. With this arrangement flow distribution in the whole system can be varied in association with the difference between the discharge pressure of the pump and the load pressure. As a consequence of the arrangement of after-orifice type pressure compensation valves having a flow dividing function, the driving speed of each actuator can be varied. However, the driving speed can only be varied at the same rate throughout the system, so that the actuators cannot be adjusted individually. The purpose of the invention is also to overcome this disadvantage. Therefore, according to the invention, the flow control system with the features known from EP-A-0 437 621, is characterized in that each of said plural valve means further comprises a correcting variable restrictor for correcting a pressure difference across said operating variable restrictor, the opening of each of said pressure compensation valves being independently set by a drive force in a closing direction based on a maximum load pressure to the corresponding actuator, a drive force in the closing direction for initial setting and a drive force in an opening direction based on a downstream pressure of the corresponding restrictor group so that the downstream pressure of the corresponding restrictor group has a constant value sufficient to drive the corresponding actuator under the maximum load pressure. In each of the above control systems, the operating variable restrictors can be arranged in flow passages for directional control valves which make it possible to drive the corresponding actuators in either a normal or reversed direction. Where the actuators are those of a hydraulic work vehicle, the operating variable restrictors can be controlled by a control lever of the hydraulic work vehicle. The above construction makes it possible to correct a pressure difference across each operating variable restrictor by fully opening its corresponding correcting variable restrictor or to ensure a limited flow rate by reducting the opening of each correcting variable restrictor. This permits a correction to fixed flow rate characteristics as needed. More detailed description of further related art A hydraulic control system for an apparatus with plural actuators driven by an oil pressure at the same time, for example, for a work vehicle such as a hydraulic excavator is equipped with a hydraulic pump, plural actuators driven by pressure oil fed from the hydraulic pump, and plural valve devices for controlling the flow rates of pressure oil to be fed from the hydraulic pump to the respective actuators. Known as hydraulic control systems of this type include, for example, load sensing control for controlling the delivery pressure of a hydraulic pump in response to a load pressure. One example of the load sensing control for controlling the delivery pressure of a hydraulic pump in response to a load pressure. One example of the load sensing control is disclosed in WO 90/00683. According to this conventional technology, there is provided pump control means for controlling the displacement of a hydraulic pump so that the delivery pressure of the hydraulic pump becomes higher by a predetermined value than a maximum load pressure to plural actuators. This pump control means is constructed of flow rate control valves provided with variable restrictors for changing the openings of plural valve devices in accordance with control signals from a control lever device, respectively, and pressure compensation valves (auxiliary valves) arranged on upper sides and in series with the respective variable restrictors to control pressure differences across the variable restrictors, respectively. By controlling the pressure differences across the respective variable restrictors with the corresponding pressure compensation valves, it has made it possible, upon combined operation to drive two or more of the actuators, to surely feed pressure oil to all the driven actuators irrespective of the levels of loads to the driven actuators so that the two or more of the actuators can be driven at the same time. To ensure smooth drive of all the actuators to be driven in parallel in such a hydraulic circuit as described above, it is necessary to constantly feed a hydraulic pressure sufficient to drive one of the actuators, to which a greatest load is applied. To this end, a control method, that is, the above-described load sensing control is adopted. This load sensing control means the control method that in a hydraulic circuit with actuators driven in parallel, the maximum load pressure out of load pressures under which the actuators are driven, respectively, in parallel is detected and the delivery capacity of a hydraulic pump is controlled to make the delivery pressure of the hydraulic pump higher by a predetermined value than the maximum load pressure. The adoption of this control method has made it possible not only to supply a sufficient hydraulic pressure to each actuator but also to allow the hydraulic pump to always feed a hydraulic pressure at a necessary level and hence to minimize power consumption. A flow control system designed to perform the above-described load sensing control can feed sufficient pressure oil to each of plural actuators to surely drive the plural actuators in parallel. It can however obtained only fixed flow rate characteristics because the flow rate of pressure oil to be fed is dependent on a maximum load pressure. The driving speed is therefore determined in a wholesale manner by the above-described maximum load pressure so that, when the flow control system is employed to control plural actuators of different types, the flow control system is unable to provide flow rate characteristics conforming with the individual actuators in view of their functions and characteristics. To make it possible to obtain flow rate characteristics conforming with the individual actuators, it has heretofore been necessary for the individual actuators to design operating variable restrictors with different restricted opening characteristics and to retain flow control systems of different ratings. Taking a hydraulic excavator, for example, cylinders as actuators from a boom, an arm and a bucket are designed to be driven at appropriate speeds, respectively, because the driven elements play different roles and their preferred drive speeds are different. It is therefore necessary to mount a variety of flow control systems conforming with the characteristics of such driven elements. This has led to problems in production efficiency and production management. In a hydraulic work vehicle driven to perform combined work at the same time, operation is not simple because the types of the work are not the same but vary substantially, because it is preferred in some instances to drive each actuator at a standard speed determined based on a stroke of the control lever and in some other instances to perform the work at a drive speed controlled lower than the standard speed. The former operation is intended to improve the efficiency of work, while the latter operation is intended to improve the accuracy of the work or the controllability. Although it is necessary for hydraulic work vehicles of this type to improve the efficiency of work and also the controllability, the conventional flow control systems which can provide only fixed flow rate characteristics are difficult to meet these two demands. Hydraulic excavators, for example, are used in increasingly diversified ways and are equipped with an increasing number of attachments in recent years due to the diversification and complication of their working sites and enlargement of their application fields. A substantial progress has hence been made in providing hydraulic excavators with many functions. As a result, a majority of work done by a hydraulic excavator has changed from standard work having importance in the efficiency of work such as excavation of the ground or loading of earth or sand so excavated to more complex work or work requiring accuracy. It is hence required to improve not only the work efficiency but also the controllability. It has hence become more important to improve the fine controllability so that the drive speed of each actuator can be increased or decreased little by little while maintaining the drive speed at a reduced speed through a control lever. To achieve this, it is necessary to enlarge an operation range of the control lever, in which fine control of the control lever is feasible, by reducing an increase or decrease in the drive speed of the actuator per unit stroke of the control lever. The conventional flow control systems, which are designed primarily for standard work and can obtain only fixed flow rate characteristics, however cannot meet such a need. Besides the flow control system described above, the technique disclosed in WO 92/04505 is also known as a flow control system of the above-described type. SUMMARY OF THE INVENTION With the foregoing technical background in view, the present invention has as a first object the provision of a flow control system which can easily obtain flow rate characteristics conforming with various actuators by making correction to fixed flow rate characteristics. A second object of the present invention is to provide a flow control system which permits improvements in the working efficiency and controllability by making it possible to easily obtain flow rate characteristics conforming with various actuators. A third object of the present invention is to provide a flow control system which permits common use of parts of various hydraulic work vehicles by making it possible to derive characteristics of various actuators with the parts of the same characteristics. To attain the above objects, the present invention provides a flow control system arranged in a hydraulic circuit provided with a variable displacement hydraulic pump, plural actuators driven by pressure oil fed from said hydraulic pressure and pump control means for controlling the displacement of said hydraulic pump so that a delivery pressure of said hydraulic pump becomes higher by a predetermined value than a maximum load pressure to said plural actuators, whereby the flow rate of the pressure oil to be fed to said actuators is controlled, comprising plural valve means connected between said hydraulic pump and said actuators, respectively, each of said plural valve means having an operating variable restrictor, whose opening can be varied by control means to control the flow rate of the pressure oil to be fed to the corresponding actuator, and a correcting variable restrictor for correcting a pressure difference across said operating variable restrictor; and pressure compensation valves arranged on upstream sides as viewed in pressure oil feeding directions of restrictor groups disposed corresponding to said actuators and having said operating variable restrictors and said correcting variable restrictors, respectively, the opening of each of said pressure compensation valves being independently set by a drive force in an opening direction based on a downstream pressure of the corresponding restrictor group, a drive force in the opening direction set by pressure-difference setting means and a drive force in a closing direction based on an upstream pressure of the corresponding restrictor group so that the upstream pressure of the corresponding restrictor group becomes higher by a predetermined pressure than the downstream pressure of the corresponding restrictor group. A similar control system can also comprise plural valve means connected between said hydraulic pump and said actuators, respectively, each of said plural valve means having an operating variable restrictor, whose opening can be varied by control means to control the flow rate of the pressure oil to be fed to the corresponding actuator, and a correcting variable restrictor for correcting a pressure difference across said operating variable restrictor; and pressure compensation valves arranged on downstream sides as viewed in pressure oil feeding directions of restrictor groups disposed corresponding to said actuators and having said operating variable restrictors and said correcting variable restrictors, respectively, the opening of each of said pressure compensation valves being independently set by a drive force in a closing direction based on a maximum load pressure to the corresponding actuator, a drive force in the closing direction for initial setting and a drive force in an opening direction based on a downstream pressure of the corresponding restrictor group so that the downstream pressure of the corresponding restrictor group has a constant value sufficient to drive the corresponding actuator under the maximum load pressure. In each of the above control systems, the operating variable restrictors can be arranged in flow passages for directional control valves which make it possible to drive the corresponding actuators in either a normal or reversed direction. Where the actuators are those of a hydraulic work vehicle, the operating variable restrictors can be controlled by a control lever of the hydraulic work vehicle. The above construction makes it possible to correct a pressure difference across each operating variable restrictor by adjusting the restricted opening of the corresponding correcting variable restrictor so that an effective pressure difference can be reduced by an appropriate value. As a result, it has become possible to ensure a large flow rate close to a maximum flow rate of each operating variable restrictor by fully opening its corresponding correcting variable restrictor or to ensure a limited flow rate by reducing the opening of each correcting variable restrictor. This permits a correction to fixed flow rate characteristics as needed. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a hydraulic circuit for load sensing control, which is equipped with a flow control system according to a first embodiment of the present invention; FIG. 2 is a hydraulic circuit for load sensing control, which is equipped with a flow control system according to a second embodiment of the present invention; FIG. 3 is a hydraulic circuit for load sensing control, which is equipped with a flow control system according to a third embodiment of the present invention; FIG. 4 is a hydraulic circuit for load sensing control, which is equipped with a flow control system according to a fourth embodiment of the present invention; FIG. 5 is a hydraulic circuit for load sensing control, which is equipped with a flow control system according to a fifth embodiment of the present invention; and FIG 6 is a characteristic diagram showing characteristics of a pressure difference across an operating variable restrictor in a flow control system according to the present invention. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS (First Embodiment) FIG. 1 shows the hydraulic circuit equipped with the flow control system according to the first embodiment of the present invention. The hydraulic circuit shown in FIG. 1 is basically constructed of a variable displacement hydraulic pump (hereinafter simply called "the pump") 1 whose delivery capacity is controlled by a tilting control unit 2, a main line 3 connected to a delivery port of the pump 1, first and second branch lines 3a,3b branched out from the main line 3, first and second load line 4a,4b connected to these branched lines 3a,3b, respectively, first and second actuators 5a,5b connected to the respective load lines 4a,4b and driven by pressure oil fed from the pump 1, first and second control units 6a,6b for controlling the flow rates of the pressure oils to be fed to the respective actuators 5a,5b, a load pressure detection line 7 for guiding to the tilting control unit 2 the pressure of the higher pressure oil out of the pressure oils fed to the respective actuators 5a,5b, and a pressure control valve 8 for reducing the pressure of the pressure oil, which has been introduced into the main line 3, in accordance with the pressure detected by the load pressure detector 7. In the description to be made hereinbelow, like reference numerals indicate like constituent elements in each branch line and the elements in each branch line are added with the same alphabetic letter as a subscript so that elements of the same type are correlated across the individual branch lines. The pressure oil is guided from the pump 1 to the first and second flow control units 6a,6b via the first and second branch lines 3a,3b. The pressure oil is fed further to the first and second actuators 5a,5b via the first and second load lines 4a,4b to independently operate the actuators 5a,5b. The flow rate control units 6a,6b are structurally identical to each other and arranged in parallel with each other. The first flow control unit 6a is constructed of a first correcting and operating variable restrictors 10a,11a arranged in series and a first pressure compensation valve 9a arranged corresponding to, upstream of and in series with the variable constrictors lOa,lla to control pressure differences across the variable restrictors lOa,lla, respectively, and acting as an auxiliary valve for auxiliarily controlling the flow rate of the pressure oil to be fed to the first actuator 5a. The second flow control unit 6b is constructed of a second correcting and operating variable restrictors 10b,11b arranged in series and a second pressure compensation restrictor 9b arranged corresponding to, upstream of and in series with the variable constrictors 10b,11b to control pressure differences across the variable restrictors lOb,llb, respectively, and acting as an auxiliary valve for auxiliarily controlling the flow rate of the pressure oil to be fed to the second actuator 5b. Of the variable restrictors, the first correcting variable restrictor 10a and the first operating variable restrictor 11a are arranged in the first branch line 3a on a downstream side of the first pressure compensation valve 9a, while the second correcting variable restrictor 10b and the second operating variable restrictor 11b are disposed in the second branch line 3b on a downstream side of the second pressure compensation valve 9b. The first and second pressure compensation valves 9a,9b are provided with a first and second closing drive units 14a,14b and a first and second opening drive units 16a,16b. An inlet and outlet pressures of the variable restrictors 10a,11a arranged in series are guided to the first closing and opening drive units 14a,16a, respectively, so that on the basis of a pressure difference across the variable restrictors 10a and 11a, the first closing drive unit 14a applies a drive force in a valve-closing direction to a spool of the first pressure compensation valve 9a while the first opening drive unit 16a applies, together with a spring 15a, a drive force in a valve-opening direction to the spool of the first pressure compensation valve 9a. An inlet and outlet pressures of the variable restrictors 10b,11b arranged in series are guided to the second closing and opening drive units 14b,16b, respectively, so that on the basis of a pressure difference across the variable restrictors 10b and 11b, the second closing drive unit 14b applies a drive force in a valve-closing direction to a spool of the second pressure compensation valve 9b while the second opening drive unit 16b applies, together with a spring 15b, a drive force in a valve-opening direction to the spool of the second pressure compensation valve 9b. To the closing drive units 14a,14b, pilot pressures are guided via pilot lines 17a,17b from upstream sides of the correcting variable restrictors 10a,10b, respectively. To the opening drive units 16a,16b, the pilot pressures are guided via pilot lines 18a,18b from downstream sides of the operating variable constrictors 11a,11b, respectively. The pressure control valve 8 is a valve which can also be called "an unloading valve", and is arranged on an upstream side of a portion of the main line 3 at which portion the main line bifurcates into the branch lines 3a,3b. When the delivery pressure of the pump 1 reaches a pressure which is the sum of a maximum load pressure of the actuators 5a,5b and a preset pressure of the pressure control valve 8 preset by a spring 19, the pressure oil delivered from the pump 1 is recirculated to the tank 20 so that the pressure of the circuit is prevented from exceeding the sum. Incidentally, a delivery pressure detection line 21 for the detection of a delivery pressure from the pump 1 is connected to the tilting control unit 2, whereby the delivery pressure of the pump 1 is controlled to a pressure higher by a predetermined value, which is set by a spring 22, than a maximum load pressure guided from the load pressure detection line 7. The variable constrictors 10a,11a;10b,11b consist of flow control valves, respectively. Their openings are changed by unillustrated control means, for example, a control lever so that the flow rates can be set by restricting action as desired. The flow control units 6a,6b themselves are constructed as a compound unit of different functions by an integral and inseparable valve unit as also described in connection with the second embodiment. In the hydraulic circuit constructed as described above, an operation of the actuators 5a,5b is performed by suitably setting the openings of the variable restrictors 10a,11a;10b,11b. A description will first be made of an operation when the actuators 5a,5b are driven by controlling the operating variable restrictors 11a,11b while maintaining the correcting variable restrictors 10a,10b in their fully open positions. Now assume that the pump 1 is operated, the operating variable restrictors 11a,11b are controlled by an operator to feed pressure oils to the respective actuators 5a,5b through the main line 3, the corresponding branch lines 3a,3b and the corresponding load lines 4a,4b, and the respective actuators 5a,5b are hence driven independently and in parallel by the pressure oils, respectively. The greater one of load pressures to the individual actuators 5a,5b is guided to the tilting control unit 2 from one of the load lines 4a,4b, said one load line being on a side of the maximum load pressure, via a connecting line 12a or 12b corresponding to the one load line and then via the load pressure detection line 7. On the other hand, a delivery pressure of the pump 1 is also guided to the tilting control unit 2 from the main line 3 via the delivery pressure detection line 21, The tilting control unit 2 controls the tilting of the pump 1 on the basis of these pressure signals so that the delivery capacity of the pump 1 is decreased when the delivery pressure of the pump 1 is higher than the sum of the maximum load pressure and a predetermined value, namely, a load sensing pressure difference but is increased when the delivery pressure of the pump 1 is lower than the sum. As a result, the delivery capacity of the pump 1 is controlled to make its delivery pressure higher by a predetermined value than the maximum load pressure, whereby load sensing control is performed. When one of the actuators 5a,5b is solely driven, the load pressure to the solely-driven actuator 5a or 5b is taken as the maximum load pressure and similar control is performed. When control is performed as described above and the actuators 5a,5b are driven in parallel, the load pressures from the load lines 4a,4b are guided from the pilot lines 18a,18b to the opening drive units 16a,16b of the pressure compensation valves 9a,9b, respectively, whereas the upstream-side pressures of the correcting variable restrictors 10a,10b are guided from the pilot lines 17a,17b to the closing drive units 14a,14b, respectively. When the downstream-side pressures of the operating variable restrictors 11a,11b exceed the sums of the load pressures and the corresponding spring forces of the springs 15a,15b, respectively, the drive forces in the closing directions are correspondingly increased so that the opening areas of the pressure compensation valves 9a,9b are reduced. Immediately before their openings are closed, the upstream-side pressures of the operating variable restrictors 11a,11b are substantially equal to the sums of the corresponding load pressures and spring forces, respectively. As a result, the pressure differences across the operating variable restrictors 11a,11b have values equivalent to the values obtained by subtracting the downstream-side pressures of the corresponding operating variable restrictors 11a,11b, that is, the load pressures from the upstream-side pressers of the corresponding operating variable restrictors 11a,11b, in other words, to the forces applied by the corresponding springs. When the load pressures increase or the upstream-side pressures of the operating variable restrictors 11a,11b drop in the above state, the drive forces applied in the opening directions to the pressure compensation valves 9a,9b, respectively, increase relative to the upstream-side pressures so lowered so that the opening areas of the pressure compensation valves 9a,9b are increased. In this manner, the openings of the pressure compensation valves 9a,9b are automatically controlled to raise the upstream-side pressures of the operating variable restrictors 11a,11b. When the load pressures or the upstream-side pressures begin to return toward the previous levels, the closing drive forces increase as opposed to the load pressures or the upstream-side pressures to reduce the opening areas of the pressure compensation valves 9a,9b, whereby the openings of the pressure compensation valves 9a,9b are automatically controlled to lower the upstream-side pressures of the operating variable restrictors 11a,11b. As is appreciated from the foregoing, the pressure compensation valves 9a,9b are automatically controlled to keep constant the pressure differences across the variable restrictors 10a,11a;10b, 11b. As a result, the pressure difference across the operating variable restrictors 11a,11b are compensated in pressure by the pressure compensation valves 9a,9b so that they always remain constant irrespective of the load pressures. Further, the operating variable restrictors 11a,11b feed pressure oils of a constant flow rate to the actuators 5a,5b in accordance with their restricted openings without being affected by the load pressures to the actuators 5a,5b. The drive speeds of the actuators 5a,5b are therefore maintained constant as long as the corresponding restricted openings remain constant. In this state, the correcting variable restrictors 10a,10b are kept open so that they are not functioning at all. Accordingly, a description will next be made of the situation that the openings of the correcting variable restrictors 10a,10b have also been changed in addition to the changes to the openings of the operating variable restrictors 11a,11b. When the openings of these correcting variable restrictors 10a,10b are changed, the pressure differences across the operating variable restrictors 11a,11b are changed. It is therefore possible to set the pressure differences across the operating variable restrictors 11a,11b at desired values by changing the restricted openings of the correcting variable restrictors 10a,10b as needed. Functions of these operating variable restrictors 11a,11b and correcting variable restrictors 10a,10b will hereinafter be described in detail. As the openings of the pressure compensation valves 9a,9b are controlled to make the pressure differences between the upstream-side pressures of the correcting variable restrictors 10a,10b and the downstream-side pressures of the operating variable restrictors 11a,11b become equal to the resilient biasing forces of the springs 15a,15b, respectively, their relationship can be expressed by the following formula: a(Pz i - Pℓ i ) = k i (xo i + x i ) ∴ Pz i - Pℓ i = (xo i + x i )k i /a = ΔPo where Pz i : The upstream-side pressure of the correcting variable restrictor 10a or 10b (the secondary pressure of the pressure compensation valve 9a or 9b). Pℓ i : The downstream-side pressure of the operating variable restrictor 11a or 11b (the load pressure to the actuator 5a or 5b). a: The pressure-receiving area of the pressure compensation valve 9a or 9b for Pz i and Pℓ i . k i : The spring constant of the spring 15a or 15b. xo i : The displacement of the spring 15a or 15b given upon initial setting. x i : The displacement of the spring 15a or 15b occurred upon application of a controlling force. Po i : The pressure difference between the upstream-side pressure of the correcting variable restrictor 10a or 10b and the downstream-side pressure of the operating variable restrictor 11a or 11b. On the other hand, the delivery pressure Ps of the pump 1 which feeds the pressure oil to the pressure compensation valves 9a,9b on primary sides thereof is controlled by load sensing control so that the delivery pressure Ps becomes higher than a maximum load pressure Ptmax by a predetermined value, namely, a load sensing pressure difference ΔP LS . The delivery pressure Ps of the variable displacement hydraulic pump 1 is controlled to always provide the constant load sensing pressure difference ΔP LS owing to the load sensing control as indicated by the following formula: Ps = Pℓmax + ΔP LS This load sensing pressure difference ΔP LS is set to substantially satisfy the following formula: ΔP LS = ΔPO i that is, to become equal to the pressure difference ΔPo i between the upstream-side pressure of the correcting variable restrictor 10a or 10b and the downstream-side pressure of the operating variable restrictor 11a or 11b (hereinafter called "the pressure difference across the restrictor"). As is evident from these formulas (2) and (3), each pressure compensation valve 9a or 9b performs control so that the pressure difference ΔPo i across the restrictor at a substantially constant value which is equal to the resilient biasing force of the spring 15a or 15b. Since the load sensing pressure difference ΔP LS is set equal to the biasing force, the pressure compensation valve 9a or 9b performs control so that as a matter of fact, the pressure difference ΔPo i across the restrictor is held at a constant value substantially equal to the load sensing pressure difference ΔP LS . The relationship among this pressure difference ΔPo i across the restrictor, the pressure difference ΔPv i across the operating variable restrictor 11a or 11b and the pressure difference ΔPm i across the correcting variable restrictor 10a or 10b can be expressed by the following formula: ΔPo i = ΔPv i + ΔPm i From these formulas (3) and (4), the pressure difference ΔPm i across the correcting variable restrictor 10a or 10b can be expressed using the pressure difference ΔPv i across the operating variable restrictor 11a or 11b as shown by the following formula: ΔPm i = ΔPo i - ΔPv i = ΔP LS - ΔPv i Further, the relationships between the pressure differences ΔPv i ,ΔPm i across the operating variable restrictor 11a or 11b and the correcting variable restrictor 10a or 10b and flow rates Qv i ,Qm i across the operating variable restrictor 11a or 11b and the correcting variable restrictor 10a or 10b can be expressed by the following formulas: Qv i = N·A i (ΔPv i ) Qm i = N·B i (ΔPm i ) where Qv i : The flow rate through the operating variable restrictor 11a or 11b. Qm i : The flow rate through the correcting variable restrictor 10a or 10b. A i : The restricted opening area of the operating variable restrictor 11a or 11b. B i : The restricted opening area of the operating variable restrictor 10a or 10b. N: Constant. The flow rate Qv i across the operating variable restrictor 11a or 11b and the flow rate Qm i through the correcting variable restrictor 10a or 10b are flow rates of the pressure oil flowing through the same flow passage and are hence equal to each other. The following formula can therefore be derived form the formulas (5), (6) and (7). As a consequence, the pressure difference Pv i across each operating variable restrictor 11a or 11b can be expressed as follows: N·A i (ΔPv i ) = N·B i (ΔPm i ) = N·B i ΔP LS - ΔPv i ) ∴ Pv i = (B i /A i ) 2 /{1 + (B i /A i ) 2 }·(ΔP LS ) = X i 2 /(1 + X i 2 ))·ΔP LS Incidentally, X i means the opening area ratio (B i /A i ) of the correction variable restrictor 10a or 10b to the operating variable restrictor 11a or 11b. Further, the flow rage Qv i through the operating variable restrictor 11a or 11b is expressed as follows: Qv i = N·A i (ΔPv i ) = N·A i {X i 2 /(1 + X i 2 )·ΔP LS } = {X i 2 /(1 + X i 2 )} ·N·A i (ΔP LS ) The relationship represented by the above formula (8), that is, the characteristics of how the pressure difference ΔPv i across the operating variable restrictor 11a or 11b changes depending on the opening area ratio X i can be illustrated as shown in FIG. 6. The characteristics of the operating variable restrictor 11a or 11b illustrated in FIG. 6 generally indicate that the difference ΔPv i across the operating variable restrictor 11a or 11b becomes greater to have a value closer to the load sensing pressure difference ΔPls (i.e., the compensating pressure by the pressure compensation valve 9a or 9b) as the ratio of the opening area B i of the correcting variable restrictor 10a or 10b to the opening area A i of the operating variable restrictor 11a or 11b becomes greater and also that as the ratio becomes smaller, the effective pressure difference across the operating variable restrictor 22 or 23 decreases and the pressure difference ΔPv i across the operating variable restrictor 22 or 23 becomes smaller. According to this embodiment, the opening area ratio X i at the time of full opening of the operating variable restrictor 11a or 11b is set at a sufficiently large value by performing control so that the restricted opening of the correcting variable restrictor 10a or 10b becomes sufficiently large relative to the restricted opening of the operating variable restrictor 11a or 11b at the time of full opening (i.e., when the control lever is moved over the entire stroke). This has made it possible to surely maintain such a large flow rate as that available when the operating variable restrictors 10a,10b are not arranged. Further, by setting the opening area ratio X i at the time of full opening of the operating variable restrictor 11a or 11b smaller than the value set above at the sufficiently large value to an extent not causing any abrupt change in the pressure difference across the operating variable restrictor 11a or 11b even when the operated quantity of the operating variable restrictor 11a or 11b is changed, a rather limited flow rate can be secured in accordance with the opening area ratio X i so set. Adequate setting of the restricted opening of the correcting variable restrictor 10a or 10b within such limits that no abrupt change would take place in the pressure difference across the restrictor makes it possible to freely adjust within a predetermined range the flow rate of pressure oil to be fed to the actuators 5a,5b during standard work. As a result, it is possible to make corrections to fixed flow rate characteristics of the conventional flow control systems not equipped with the correcting variable restrictors 10a,10b so that flow rate characteristics conforming with various kinds of actuators can be readily obtained. Further, by performing control so that the ratio of the open area of the correcting variable restrictor 10a or 10b to the open area of the operating variable restrictor lla or llb at the time of full opening becomes smaller than the sufficiently large opening area ratio X i , the drive speed of the actuator 5a or 5b per unit stroke of the control lever can be kept slow. This has made it possible to expand the operation range of the shift lever in which fine control can be performed, thereby improving the controllability, in this case, as the opening area ratio X i is made smaller, the pressure difference across the operating variable restrictor lla or llb undergoes a greater change per unit stroke of the control lever. As the stroke of the control lever is made greater, the flow rate increases although the percent increment in the flow rate becomes smaller. As a result, an increase in the drive speed of the actuator 5a or 5b per unit stroke is more limited as the drive speed becomes higher. This means that the drive speed can be adjusted by still smaller degrees in a relative large range. As a consequence, the control range of the control lever, in which fine control can be performed, can be enlarged under ideal conditions, thereby making it possible to obtain a flow control system suitable especially for work which is complex and requires accuracy. The above-described characteristics which have made it possible to prevent the drive speed of the actuator 5a or 5b from increasing significantly even when the control lever is moved over a large stroke can also be used for an operation in which no quick drive of the actuator 5a or 5b is desired, for example, for lowering a boom. Appropriate use of the above characteristics in various stages of work permit use of the flow control system for diversified applications. Adding further, if it is desired to have the drive speed of the actuator 5a or 5b always varied in proportion to a stroke of the control lever in various work, it is only necessary to design in such a way that the opening area ratio X i can be always maintained constant by having the restricted opening of the correcting variable restrictor 10a or 10b associated with the restricted opening of the operating variable restrictor 11a or 11b, for example, by controlling these variable restrictors under the control of a pilot pressure. In the flow control system according to this embodiment, the correcting variable restrictors 10a,10b are arranged on the upstream sides of the operating variable restrictors 11a,11b, respectively, so that two restrictor groups, each consisting of the two restrictors, are formed. Insofar as the arrangement of the pressure compensation valves 9a,9b relative to these restrictor groups meet such a relationship as in this embodiment, absolutely no influence takes place to the formulas described above even if the positional relationship between the correcting variable restrictors 10a or 10b and the operating variable restrictor 11a or 11b is made opposite. It is therefore clear that no variations occur in the characteristics of the flow control system no matter how their positional relationship is. Accordingly, similar functions can be exhibited even if the positional relationship between the correcting variable restrictor 10a or 10b and the operating variable restrictor 11a or 11b is changed as needed. (Second Embodiment) The hydraulic circuit according to the second embodiment is illustrated in FIG. 2. This embodiment is different from the above-described first embodiment in that the first and second actuators 5a,5b can be driven in both directions. This bidirectional drive has been enabled by directional control valves. This embodiment is characterized in that the actuators 5a,5b are provided with load lines 4a-1,4a-2;4b-1,4b-2, respectively, and the drive directions and speeds of the actuators 5a,5b can be controlled by a first and second directional control valves 41a,41b. The remaining elements are equivalent to the corresponding elements of the first embodiment so that such equivalent elements are identified by like reference numerals. A description will therefore be made of different elements only. In FIG. 2, the load lines 4a-1,4b-1 are connected to bottom-side compartments of the actuators 5a,5b while the load lines 4a-2,4b-2 are connected to rod-side compartments of the actuators 5a,5b. The load lines 4a-1,4a-2 are each connected to the first directional control valve 41a while the load lines 4b-1,4b-2 are each connected to the second directional control valve 41b. The directional control valves 41a,41b are connected to an unillustrated hydraulic pilot device. In the directional control valve 41a, pilot pressures P 1 ,P 2 from the hydraulic pilot device operated by a control lever (not shown) are fed to pilot terminals 42a,43a, respectively, so that the directional change-over and opening of the directional control valve 41a are controlled. In the directional control valve 41b, on the other hand, pilot pressures from the hydraulic pilot device operated by the unillustrated control lever are similarly fed to pilot terminals 42b,43b so that the directional change-over and opening of the directional control valve 41b are performed. The directional control valves 41a,41b are internally provided with operating variable restrictors 44a-1,44a-2;44b-1,44b-2 having similar functions to the operating variable restrictors lla,llb in the first embodiment described above. When the control lever of the unillustrated hydraulic pilot control device is operated to drive a piston rod of the first actuator 5a in an advancing direction, the first directional control valve 41a is switched by a pilot pressure P 1 from a center valve position to a left position as viewed in the drawing. When the control lever is operated to drive the piston rod of the first actuator 5a in a retreating direction, the first directional control valve 41a is switched by a pilot pressure P 2 from the center valve position to a right position as viewed in the drawing. Likewise, the second directional control valve 41b is switched by a pilot pressure P 3 from a neutral valve position to a left position as viewed in the drawing when the control lever is operated to drive a piston rod of the second actuator 5b in an advancing direction, and is switched by a pilot pressure P 4 from the neutral valve position to a right position when the control lever is operated to drive the piston rod of the second actuator 5b in a retreating direction. The directional control valves 41a,41b, when switched to the left positions, feed the pressure oils from the load lines 4a-1,4b-1 to the bottom-side compartment of the actuators 5a,5b via the operating variable restrictors 44a-1,44b-1, respectively, and have the other load lines 4a-2,4b-2 communicated to corresponding tank ports to release the pressure oils from the rod-side compartments of the respective actuators 5a,5b to the tank 20, whereby the actuators 5a,5b are driven upward. When switched to the right positions, the directional control valves 41a,41b feed the pressure oils from the respective load lines 4a-2,4b-2 to the rod-side compartments of the actuators 5a,5b via the operating variable restrictors 44a-2,44b-2, respectively, and have the other load lines 4a-2,4b-2 communicated to corresponding tank ports to release the pressure oils from the bottom-side compartments of the respective actuators 5a,5b to the tank 20, whereby the actuators 5a,5b are driven downward. At this time, displacements of spools of the directional control valves 41a,41b are controlled in accordance with the values of the pilot pressures P 1 ,P 2 ,P 3 ,P 4 outputted by the hydraulic pilot control device, in other words, by the strokes of the control lever so that the restricted openings of the individual variable restrictors 44a-1,44a-2;44b-1,44b-2 are set. Further, a flow control valve 45a is arranged in the branch line 3a on the downstream side of the first pressure compensation valve 9a so that the rate of a flow to the first directional control vale 41a can be adjusted. One of ports of the flow control valve 45a is provided with a correcting variable restrictor 47a so that a pressure difference across the first directional control valve 41a can be corrected. The other port can feed the pressure oil to the directional control valve 41a without any restriction. Further, to a pilot terminal 46a at one (restricted) portion of the flow control valve 45a, the higher one of the pilot pressures P 3 ,P 4 to be guided to the second directional control valve 41b arranged in the branch line 3b is guided as a pilot pressure P 5 . A spool of the flow control valve 45a is displaced in accordance with the balancing in drive force between the pilot pressure P 5 and the spring force of a spring attached to an opposite port to regulate the initial setting pressure, whereby the restricted opening of the variable restrictor 47a is set. In this embodiment, in the course of driving the actuators 5a,5b through the directional control valves 41a,41b, the spool of the flow control valve 45a is displaced to change the opening of the correcting variable restrictor 47a, whereby the pressure difference across the restrictor of the directional control valve 41a can be corrected. With respect to the actuator 5a, the characteristics of the operating variable restrictors 44a-1,44a-2 are changed as in the first embodiment so that characteristics suited for work or operation can be selected. Incidentally, the operating system of the actuator 5b in the second embodiment is not provided with any correcting variable restrictor and the restricted opening is set by the pilot pressures for the switching control of the directional control valve 41b. The hydraulic circuit is therefore driven with priority on the side of the actuator 5b. In FIG. 2, the initial setting pressure of the flow control valve 45a is set by the spring 48a, and the correcting variable restrictor 47a functions only when the pilot pressure P 5 has exceeded the initial setting pressure of the spring 48a. Other elements which have not been described specifically are constructed as in the first embodiment and exhibit similar effects. (Third Embodiment) The hydraulic circuit according to the third embodiment is illustrated in FIG. 3. This embodiment is similar to the second embodiment except for the additional arrangement of a variable restrictor in the second branch line 3b so that the variable restrictor can be used for the drive and control of the second actuator 5b. In this embodiment, too, only elements different from the first and second embodiments will be described, and elements considered to be equivalent to their corresponding elements in the first and second embodiments will be identified by like reference numerals and their description is omitted herein. As is understood from FIG. 3, this embodiment is provided with a second flow control valve 45b, which is similar to the first flow control vale 45a arranged in the branch line 3a, in the branch line 3b between the second pressure compensation valve 15b and the second directional control valve 41b, pilot pressure feed lines 49a,49b connected to the pilot terminal 46a of the first flow control valve 45a and the pilot terminal 46b of the second flow control valve 45b to feed pilot pressures P 6 ,P 7 , and a pilot pressure control unit 50 for controlling these pilot pressures P 6 ,P 7 . The remaining elements are constructed as in the second embodiment shown in FIG. 2. Incidentally, the pilot control unit 50 is constructed of a solenoid-operated proportional pressure reducing valve 51 and a controller 54. The solenoid-operated proportional pressure reducing valve 51 feeds the pressure oil from an oil pressure source 52 to the pilot lines 49a,49b after reducing the pressure of the pressure oil, whereas the controller 54 controls the opening of the solenoid-operated proportional pressure reducing vale 51 in accordance with a command from a speed-adjusting dial 53 to change each pilot pressure. Owing to this construction, the operator can change pilot pressures P 6 ,P 7 by controlling the speed-adjusting dial 53. The openings of the correcting variable restrictors 47a,47b then vary in accordance with the pilot pressures P 6 ,P 7 so changed, whereby the pressure differences across the operating variable restrictors 44a-1,44a-2;44b-1,44b-2 of the directional control valves 41a,41b can be set as needed. In this embodiment, it is designed to simultaneously change the pilot pressures P 6 ,P 7 by the single speed-adjusting dial 53. It is however possible to arrange two speed-adjusting dials so that the pilot pressures P 6 ,P 7 can be separately set to independently set the openings of the correcting variable restrictors 47a,47b. This matter should be determined as needed depending on the performance and application of a hydraulic equipment in which the hydraulic control system is used. Other elements which have not been described specifically are constructed as in the first and second embodiments and exhibit similar effects to those in the first and second embodiments. (Fourth Embodiment) The hydraulic circuit according to the fourth embodiment is illustrated in FIG. 4. As this embodiment is constructed in a similar manner to the above-described first embodiment except for the first and second flow control units 9a,9b, a description will be made of flow control units only. Like elements to the corresponding ones in the first embodiment are identified by like reference numerals, and any overlapping description is omitted herein. In FIG. 4, a first and second flow control units 30a,30b are constructed of a first and second correcting variable restrictors 31a,31b, a first and second operating variable restrictors 32a,32b and a first and second pressure compensation valves 33a,33b, respectively. These restrictors and valves are arranged in the first and second branch lines 3a,3b in the order they are presented from the upstream sides as viewed in the feeding direction of pressure oil. The pressure compensation valves 33a,33b are provided with closing drive units 35a,35b for applying a drive force, which is based on a maximum load pressure, in valve-closing directions to them together with spring forces of springs 34a,34b and also with opening drive units 36a,36b for applying drive forces in valve-opening directions. Pilot pressures are guided from downstream sides of a first and second check valves 13a,13b to the closing drive units 35a,35b via pilot lines 37a,37b, respectively, while pilots pressures are guided from downstream sides of the first and second operating variable restrictors 32a,32b to the opening drive units 36a,36b via pilot lines 38a,38b, respectively. A control mechanism and the like for the variable restrictors 31a,32a;31b,32b, including matters not specifically described here, are all constructed as in the first embodiment. In this embodiment, the flow control units are also constructed as a compound unit having different functions, namely, inseparably as a valve unit. In the fourth embodiment constructed as described above, the actuators 5a,5b are operated by setting the openings of the variable restrictors 31a,32a;31b,32b of the hydraulic circuit as needed like the first embodiment. With respect to this embodiment, a first description will also be made of an operation when the actuators 5a,5b are driven by controlling only the operating variable restrictors 32a,32b while maintaining the correcting variable restrictors 31a,32a in the fully-opened positions. When the actuators 5a,5b are driven in parallel, a maximum load pressure in the load pressure detection line 7 is guided to the closing drive units 35a,35b of the pressure compensation valves 33a,33b via the pilot lines 37a,37b, respectively. On the other hand, downstream pressures of the operating variable restrictors 32a,32b are guided to the closing drive units 36a,36b of the pressure compensation vales 33a,33b via the pilot lines 38a,38b so that drive forces are applied in the valve-opening directions, respectively. When the downstream pressures of the operating variable restrictors 32a,32b become higher than predetermined values set by the maximum load pressure and the spring forces of the springs 34a,34b, the opening drive forces are increased correspondingly to enlarge the openings of the pressure compensation valves 33a,33b, whereby hydraulic pressures of values sufficient to drive the actuators 5a,5b are hence fed to the actuators 5a,5b. If the maximum load pressure begins to drop or the downstream pressures of the operating variable restrictors 32a,32b begin to become still higher under the above situation, the pressure compensation valves 33a,33b increase their flow passage areas further so that their openings are self-controlled to lower the downstream pressures. If the maximum load pressure begins to increase or the downstream pressures begin to drop conversely, the pressure compensation valves 33a,33b decrease their flow passage areas so that their openings are self-controlled to increase the downstream pressures. As a consequence, the downstream pressures of the operating variable restrictors 32a,32b are always maintained at levels somewhat higher than the maximum load pressure owing to such a pressure-regulating function of the pressure compensation valves 33a,33b without being affected by variations in the circuit pressure such as the load pressures to the actuators 5a,5b or the delivery pressure of the pump 1. In other words, the pressure differcnces across the operating variable restrictors 32a,32b are compensated in pressure to always remain constant without being affected by variations in the openings of the operating variable restrictors 32a,32b or the loads to the actuators 5a,5b, also owing in part to the control of the delivery pressure of the pump 1 in accordance with the load sensing control. As a result, the operating variable restrictors 32a,32b can be set at constant flow rates corresponding to their restricted openings without being affected by variations in the pressure of the circuit. In the above state, the correcting variable restrictors 31a,31b are, as described above, kept open and not functioning at all. When the openings of the correcting variable restrictors 31a,31b are also changed in addition to the operating variable restrictors 32a,32b, the pressure differences across the operating variable restrictors 32a,32b also change. By changing the restricted openings of the correcting variable restrictors 31a,31b as needed, the pressure differences across the operating variable restrictors 32a,32b can therefore be set at desired values. This characteristic will hereinafter be described using formulas. With respect to the pressure compensation valves 33a,33b, first, the downstream pressure of each operating variable restrictors 32a or 32b arranged on the upstream side of the pressure compensation valves 33a or 33b, in other words, the downstream pressure Pz i of each restrictor group can be expressed by the following formula: Pz i = Pℓmax + k i /a(Zo i + Z i ) = Pℓmax + Co i where Pz i : The downstream pressure of the operating variable restrictor 32a or 32b (the primary pressure of the pressure compensation valve 33a or 33b). Pℓmax: The maximum load pressure. a: The pressure-receiving area of the pressure compensation valve 33a or 33b for Ptmax and Pz i . k i : The spring constant of the spring 34a or 34b. Zo i : The displacement of the spring 34a or 34b applied upon initial setting. Z i : The displacement of the spring 34a or 34b upon application of a control force. Co i : constant. In the above formula (10), Co i stands for the resilient biasing force k i /a(Zo i + Z i ) of the spring 34a or 34b as the resilient biasing force can be considered as a constant. The resilient biasing force of the spring 34a or 34b is to apply a small displacement upon initial setting to set the pressure compensation valve 33a or 33b in the closed position, so that the downstream pressure of the operating variable restrictor 32a or 32b can be controlled at a constant value somewhat higher than the maximum load pressure. Since the resilient biasing force is adjusted to a practically-ignorable very small value, the pressure compensation valve 33a or 33b is considered to control the downstream pressure Pz i of the operating variable restrictor 32a or 32b at a value substantially equal to the maximum load pressure Pℓmax. On the other hand, the delivery pressure Ps of the pump 1 which feeds the pressure oil to the upstream sides of the operating variable restrictors 32a,32b is controlled at pℓmax + ΔP LS , which is higher by the load sensing pressure difference ΔP LS than the maximum load pressure Pℓmax as shown by the formula (2). This delivery pressure Ps is fed to the branch lines 3a,3b and becomes the upstream pressure of the operating variable restrictors 32a,32b. When the operating variable restrictors 32a,32b of the respective flow control units 33a,33b have been set at desired restricted openings, the pressure difference Ps - Pz i between the upstream and downstream pressures of each operating variable restrictor 32a or 32b is maintained at a constant value approximating the load sensing pressure difference ΔPls in view of the formulas (2) and (10) as indicated by the following formula (11): Ps - Pz i = (Pℓmax + ΔP LS ) - (Pℓmax + Co i ) = ΔP LS - Co i ≈ ΔP LS Namely, the pressure difference across the operating variable restrictor 32a or 32b is controlled and compensated in pressure by the pressure compensation valve 33a or 33b so that irrespective of the level of the load pressure, the pressure difference is always maintained at a pressure substantially equal to the load sensing pressure difference AP LS . As a result, the flow rate Qv i ' through the operating variable restrictor 32a or 32b is defined as follows: Qv i ' = N·A i (Ps - Pz i ) Qv i ' = N·A i (ΔP LS ) The flow rate Qv i ' therefore remains at a value proportional to a restricted opening set by the operating variable restrictor 32a or 32b without being affected by variations in the pressure of the circuit. As long as the restricted opening remains unchanged, the flow rate Qv i ' can be maintained at the constant value. Since Co i in the formula (10) is adjusted to take an ignorably small value, the downstream pressure Pz i of the restrictor group in each flow control unit 30a or 30b is controlled at a level substantially equal to the maximum load pressure ptmax. On the other hand, the delivery pressure Ps of the hydraulic pump 1 which feeds the pressure oil to the upstream side of each correcting variable restrictor 31a or 31b is controlled by the load sensing control at Pℓmax + ΔP LS , which is higher than the maximum load pressure Pℓmax by the load sensing pressure difference PLs as indicated by the formula (2). This delivery pressure Ps is delivered to each branch line 3a or 3b and becomes the upstream pressures of the correcting variable restrictor 31a or 31b. When each operating variable restrictors 32a or 32b has been set at a desired valve opening, the following formula can be established from the formulas (2) and (10) between the upstream pressure of the correcting variable restrictor 31a or 31b and the downstream pressure of the operating variable restrictor 32a or 32b, said correcting variable restrictor and said operating variable restrictor being arranged in series: Ps - Pz i = (Pℓmax + ΔP LS ) - (Pℓmax + Co i ) = ΔP LS - Co i ≈ ΔP LS This indicates that the pressure difference Ps - Pz i is always maintained at a constant value approximating the load sensing pressure difference ΔP LS . This load sensing pressure difference ΔP LS and the pressure difference ΔPo i across each restrictor group consisting of restrictors arranged in series as in this embodiment can be expressed by the pressure difference ΔPv i across the operating variable restrictor 32a or 32b and the pressure difference ΔPm i across the correcting variable restrictor 31a or 31b in a similar manner to the above-described formula (4). These pressure differences ΔPv i , ΔPm i and the flow rates through the operating variable restrictors 32a,32b and the correcting variable restrictors 31a,31b can be expressed similarly to the formulas (6) and (7) described above. The flow rate Qv i through each operating variable restrictor 32a or 32b and the flow rate Qm i through the corresponding correcting variable restrictor 31a or 31b are the flow rates of the pressure oil flowing through the same flow passage and are equal to each other. From the formulas (4), (6) and (7) described above, the pressure difference ΔPv i across the operating variable restrictor 32a or 32b can be expressed similarly to the above-described formula (8) so that the flow rate Qv i through the operating variable restrictor 32a or 32b can be derived like the above-described formula (9), that is, can be expressed as follows: Qv i = N·A i (ΔPv i ) = N·A i {X i 2 /(1 + X i 2 )·ΔP LS } = {X i 2 /(1 + X i 2 )} ·N·A i (ΔP LS ) = {X i 2 /(1 + X i 2 )}·Qv i ' The characteristics of the pressure difference ΔPv i across the operating variable restrictor 32a or 32b in each flow control unit 30a,30b of the fourth embodiment can also be illustrated as in FIG. 6 similarly to the first embodiment. In all the flow control units 6a,6b;30a,30b in the first and fourth embodiments, the opening area ratios X i become smaller and the pressure differences across the operating variable restrictors lla,llb;31a, 31b becomes smaller as the stroke of the control lever, in other words, the valve openings of the operating variable restrictors 11a,11b;32a,32b increase, provided that the restricted openings of the correcting variable restrictors 10a,10b;31a,31b are constant. This however does not mean that each flow rate decreases as the stroke of the control lever increases. It is meant that although each flow rate increases with the stroke of the control lever, the flow rate does not increase proportionally in response to the stroke but the flow rate increases with the percent increment relative to the stroke being limited. When the stroke of the control lever is increased with the restricted openings of the correcting variable restrictors 10a,10b;31a,31b being kept constant, the pressure differences across the operating variable restrictors 11a,11b;32a,32b become smaller in accordance with the stroke so that the flow rates therethrough do not necessarily increase in proportion to the stroke of the control lever. As is appreciated from FIG. 6, however, a variation in the pressure difference across each restrictor per unit stroke of the control lever is relatively small in a range where the opening area ratio X i is greater than a predetermined value (for example, 2 in FIG. 6). In such a range, the drive speed of each actuators 5a or 5b varies substantially in proportion to a stroke of the control lever, so that no particular problem or inconvenience arises in the feeling of operation during standard work by a hydraulic work vehicle. In a range where the opening area ratio X i is relatively smaller than the predetermined value, on the other hand, the pressure difference across each restrictor varies at a relatively greater rate. The flow rate however increases at a progressively increasing, limited percent increment as the stroke becomes greater. The drive speed of each actuator 5a or 5b can therefore be prevented from suddenly increasing even when the stroke of the control level is increased. Setting aside the question of whether the flow control system is suited for such work as placing importance on efficiency, appropriate use of these characteristics hence make it possible to use the flow control system conveniently for other work. (Fifth Embodiment) The hydraulic circuit according to the fifth embodiment is depicted in FIG. 5. This embodiment is characterized in that in the fourth embodiment described above, the load lines 4a-1,4a-2;4b-1,4b-2 are arranged in association with the first and second actuators 5a,5b, respectively, the driving directions and drive speeds of the actuators 5a,5b are controllable by the directional control valves 41a,41b and, upon actuation of overload relief valves, the pressure differences across the operating variable restrictors can be corrected by the corresponding correcting variable restrictors. As the other elements are constructed as in the fourth embodiment, like elements are therefore identified by like reference numerals, and only the different elements will be described. In FIG. 5, the load lines 4a-1,4b-1 are connected to the bottom sides of the first and second actuators 5a,5b and the load lines 4a-2,4b-2 are connected to their rod sides, respectively. The load lines 4a-1,4a-2 are connected to a first directional control valve 60a, while the load lines 4b-1,4b-2 are connected to a second directional control valve 60b. The first and second directional control valves 60a,60b are each connected to an unillustrated hydraulic pilot pressure device. To the pilot terminals 62a,63a of the first directional control valve 60a, pilot pressures P 1 ,P 2 are fed from the hydraulic pilot pressure device which has been operated by an unillustrated control lever, whereby the directional control valve 60a is controlled in the switching of its direction and the opening. Likewise, pilot pressures P 3 ,P 4 are fed to pilot terminals 62b,63b of the second directional control valve 60b from the hydraulic pilot pressure device which has been operated by the unillustrated control lever, so that the directional control valve 60b is controlled in the switching of its direction and the opening, The directional control valves 60a,60b are internally provided with operating variable restrictors 61a-1,61a-2;61b-1,61b-2 having similar functions to the operating variable restrictors in the fourth embodiment described above. When the control lever of the unillustrated hydraulic pilot pressure device is operated in a direction to displace a piston rod of the actuator 5a in an advancing direction, the directional control valve 60a is switched by the pilot pressure P 1 to a left position from a center valve position as viewed in the drawing. When the control lever is operated in a direction to displace the piston rod of the actuator in a retreating direction, the directional control valve 60a is switched by the pilot pressure P 2 to a right position from the center valve position. When the control lever is operated in a direction to displace a piston rod of the actuator 5b in an advancing direction, the directional control valve 60b is similarly switched by the pilot pressure P 3 to a left position from a center valve position as viewed in the drawing. When the control lever is operated in a direction to displace the piston rod of the actuator in a retreating direction, the directional control valve 60b is switched by the pilot pressure P 4 to a right position from the center valve position. When switched to the left positions, the first and second directional control valves 60a,60b feed pressure oils to the bottom sides of the respective actuators 5a,5b from the load lines 4a-1,4b-1 via the operating variable restrictors 61a-1,61b-1 and the first and second pressure compensation valves 36a,36b and at the same time, communicate the other load lines 4a-2,4b-2 to their corresponding tank ports to release the pressure oils from the rod sides of the respective actuators 5a,5b. Accordingly, the actuators 5a,5b are driven upwards. When switched to the right positions, on the other hand, the first and second directional control valves 60a,60b feed the pressure oil from the load lines 4a-2,4b-2 to the rod sides of the actuators 5a,5b via the operating variable restrictors 61a-2,61b-2 and at the same time, communicate the other load lines 4a-1,4b-1 to their corresponding tank ports to release the pressure oils from the bottom sides of the respective actuators 5a,5b. Accordingly, the actuators 5a,5b are driven downwards. At this time, spools of the directional control valves 60a,60b are controlled, like the second embodiment, in displacement according to the values of the pilot pressures P 1 ,P 2 ,P 3 ,P 4 outputted from the hydraulic pilot pressure device, namely, the stroke of the control lever, whereby the restricted openings of the variable restrictors 61a-1,61a-2;61b-1,61b-2 are set. A first and second flow control valves 64a,64b are arranged in the first and second branch lines 3a,3b on the upstream sides of the first and second directional control valves 60a,60b, so that the rates of flows to the directional control valves 60a,60b can be controlled. The flow control valves 64a,64b are provided at one ports thereof with a first and second correcting variable restrictors 65a,65b, respectively, whereby the pressure differences across the operating variable restrictors 61a-1,61a-2;61b-1,61b-2 of the directional control valves 60a,60b can be corrected. Opposite ports are formed fully open so that the pressure oils can be fed to the directional control valves 60a,60b, respectively. Further, the load lines 4a-1,4a-2;4b-1,4b-2 are provided with overload relief valves 67a-1,67a-2;67b-1,67b-2, a first restrictor 68a arranged in a discharge line from the overload relief valves 67a-1,67a-2, and a second restrictor 68b arranged in a discharge line from the overload relief valves 67b-1,67b-2. Upstream pressures of these restrictors 68a,68b are connected to pilot terminals 66a,66b of the one ports of the first and second flow control valves 64a,64b through pilot lines 69a,69b. Accordingly, pilot pressures P 8 ,P 9 are guided to the pilot terminals 66a,66b. Depending on the balancing in drive force between the pilot pressures P 8 ,P 9 and the spring forces of the springs 70a,70b attached to the other ports to specify the initial setting pressures, the spools of the flow control valves 64a,64b are displaced to set the restricted openings of the variable restrictors 65a,65b. When constructed as described above, it is possible to drive the actuators 5a,5b in both directions while still being equipped with functions similar to the fourth embodiment. Moreover, when the overload relief valves 67a-1,67a-2;67b-1,67b-2 are actuated, the openings of the correcting variable restrictors 65a,65b are set in accordance with the pressures of the pressure oils discharged from the overload relief valves 67a-1,67a-2;67b-1,67b-2 so that the pressure differences across the operating variable restrictors 61a-1,61a-2;61b-1,61b-2 are reduced. The actuators 5a,5b are therefore driven at speeds corresponding to the openings of the operating variables restrictors 61a-1,61a-2:61b-1,61b-2, which openings are determined based on the pressure differences corrected by the correcting variable restrictors 65a,65b. In addition, the flow rates of the pressure oils fed through the correcting variable restrictors 65a,65b are limited as described above. Obviously, the pressure oils to be fed to the load lines 4a-1,4a-2;4b-1,4b-2 are also limited, thereby making it possible to reduce the relief losses. Other elements not described specifically above are constructed as in the fourth embodiment and operate likewise. In the flow control units 6a,6b;40a;40a,40b; 30a,30b;59a,59b in all the first to fifth embodiments described above, the correcting variable restrictors 10a,10b;47a;47a,47b;31a,31b;65a,65b are arranged on the upstream sides of the operating variable restrictors 11a,11b;44a-1,44a-2;44a-1,44a-2,44b-1,44b-2;32a,32b; 61a-1,61a-2,61b-1,61b-2. As already described with respect to the first embodiment, it is clear that these restrictors exhibit similar effects even when their positional relationships are reversed. In essence, it is sufficient insofar as the arrangements of the pressure compensation valves 9a,9b;33a,33b relative to the restrictor groups consisting of these operating variable restrictors 11a,11b;44a-1,44a-2;44a-1,44a-2, 44b-1,44b-2;32a,32b;61a-1,61a-2,61b-1,61b-2 and these correcting variable restrictors 10a,10b;47a;47a,47b; 31a,31b;65a,65b are specified corresponding to the systems according to the individual embodiments. In these embodiments, the correcting variable restrictors 10a,10b;47a;47a,47b;31a,31b;65a,65b are singly arranged as means for correcting the pressure differences across the operating variable restrictors 11a,11b;44a-1,44a-2; 44a-1,44a-2,44b-1,44b-2;32a,32b;61a-1,61a-2,61b-1, 61b-2. It is also possible to arrange other variable restrictors or fixed restrictors in addition to these correcting variable restrictors so that the operating variable restrictors are each provided with plural correcting restrictors. In this case, the pressure differences across the operating variable restrictors 11a,11b;44a-1,44a-2;44a-1,44a-2,44b-1,44b-2;32a,32b; 61a-1,61a-2,61b-1,61b-2 are each corrected solely relying upon the corresponding plural restrictors. It is therefore evident that such modified embodiments can surely provide both large flow rates and limited flow rates like the above-described embodiments while permitting modifications to their fixed flow rate characteristics, although these flow rates may differ in extent from those available from the above-described embodiments. The above-described embodiments, when applying a control force to each pressure compensation valve, adopt the so-called hydraulic pilot control method that an upstream pressure of a corresponding restrictor group, a downstream pressure of the restrictor group and a maximum load pressure are introduced as pilot pressures. Based on these pressures, a control force can also be applied by the solenoid-operated pilot control method or the solenoid-operated control method. In essence, no particular limitation is imposed on the control method insofar as a control force is applied to each pressure compensation valve on the basis of an upstream pressure of a corresponding restrictor group, a downstream pressure of the restrictor group and a maximum load pressure. Further, a spring is employed to preset each pressure compensation valve in the embodiments described above. This objective can also be achieved similarly by using a hydraulic pilot pressure or the like. In essence, no particular limitation is imposed in this regard insofar as each pressure compensation valve is provided with means for applying thereto a desired drive force upon initial setting.

Claims: A flow control system arranged in a hydraulic circuit provided with a variable displacement hydraulic pump (1), plural actuators (5a,5b) driven by pressure oil fed from said hydraulic pump and pump control means (2) for controlling the displacement of said hydraulic pump (1) so that a delivery pressure of said hydraulic pump (1) becomes higher by a predetermined value than a maximum load pressure to said plural actuators (5a,5b), whereby the flow rate of the pressure oil to be fed to said actuators is controlled, the flow control system further comprising plural valve means between said hydraulic pump (1) and said actuators (5a,5b), respectively, and pressure compensation valves (9a,9b) arranged on upstream sides as viewed in pressure oil feeding directions of said plural valve means disposed corresponding to said actuators (5a,5b), characterized in that each of said plural valve means is provided with a restrictor group with an operating variable restrictor (lla or llb; 44a-1, 44a-2 or 44b-1, 44b-2), whose opening can be varied by control means to control the flow rate of the pressure oil to be fed to the corresponding actuator, and a correcting variable restrictor (10a or 10b; 47a or 47b) for correcting a pressure difference across said operating variable restrictor, the opening of each of said pressure compensation valves (9a,9b) being independently set by a drive force in an opening direction based on a downstream pressure of the corresponding operating variable restrictor, a drive force in the opening direction set by pressure-difference setting means (15a, 15b) and a drive force in a closing direction based on an upstream pressure of the corresponding restrictor group so that the upstream pressure of the corresponding restrictor group becomes higher by a predetermined pressure than the downstream pressure of the corresponding restrictor group. A flow control system according to claim 1, characterized in that the correcting variable restrictor (47a) is arranged on a downstream side as viewed in pressure oil feeding direction of the pressure compensation valve (9a) in a hydraulic drive route (3a) for driving at least one (5a) of said actuators and on an upstream side as viewed in pressure oil feeding direction of the corresponding operating variable restrictor (44a-1, 44a-2), whereby a pressure difference across said operating variable restrictor (44a-1, 44a-2) is corrected. A flow control system according to claim 2, characterized in that said correcting variable restrictor (47a) corresponding to said at least one actuator (5a) is fed with an opening control signal from control means for the operating variable restrictor arranged in a hydraulic drive route (3b) for the actuator (5b) not provided with said correcting variable restrictor (47a), and the restricted opening of said correcting variable restrictor (47a) is changed in accordance with the opening control signal. A flow control system according to claim 3, characterized in that the opening control signal from said control means is a pilot pressure for regulating the opening of said correcting variable restrictor (47a), and said correcting variable restrictor (47a) is set to have a smaller opening as the pilot pressure becomes higher. A flow control system according to claim 2, characterized in that it further comprises restricted opening setting means for setting the restricted opening of said correcting variable restrictor (47a), said restricted opening setting means setting the restricted opening of said correcting variable restrictor independently from the restricted opening of said operating variable restrictor (44a-1, 44a-2). A flow control system according to claim 5, characterized in that said restricted opening setting means comprises a solenoid-operated proportional pressure reducing valve (51) for setting a pilot pressure which is in turn used to set the restricted opening of said correcting variable restrictor (47a), means (53) for indicating the degree of a reduction in pressure to said solenoid-operated proportional pressure reducing valve, and a controller (54) for controlling the opening of said solenoid-operated proportional pressure reducing valve (51) in accordance with the degree of the reduction in pressure from said indicating means (53). A flow control system arranged in a hydraulic circuit provided with a variable displacement hydraulic pump (1), plural actuators (5a, 5b) driven by pressure oil fed from said hydraulic pump (1) and pump control means (2) for controlling the displacement of said hydraulic pump (1) so that a delivery pressure of said hydraulic pump (1) becomes higher by a predetermined value than a maximum load pressure of said plural actuators (5a,5b), whereby the flow rate of the pressure oil to be fed to said actuators (5a,5b) is controlled, the flow control system further comprising plural valve means connected between said hydraulic pump (1) and said actuators (5a, 5b), respectively, each of said plural valve means having an operating variable restrictor (32a or 32b; 61a-1, 61a-2 or 61b-1, 61b-2), whose opening can be varied by control means to control the flow rate of the pressure oil to be fed to the corresponding actuator (5a or 5b), and pressure compensation valves (33a or 33b) arranged on downstream sides as viewed in pressure oil feeding directions of said operating variable restrictors (32a or 32b; 61a-1, 61a-2 or 61b-1, 61b-2) disposed corresponding to said actuators (5a,5b), characterized in that each of said plural valve means further comprise a correcting variable restrictor (31a or 31b; 65a or 65b) for correcting a pressure difference across said operating variable restrictor, the opening of each of said pressure compensation valves being independently set by a drive force in a closing direction based on a maximum load pressure to the corresponding actuator (5a or 5b), a drive force (34a,34b) in the closing direction for initial setting and a drive force in an opening direction based on a downstream pressure of the corresponding restrictor group so that the downstream pressure of the corresponding restrictor group has a constant value sufficient to drive the corresponding actuator (5a or 5b) under the maximum load pressure. A flow control system according to claim 7, characterized in that it further comprises overload relief valves (67a-1, 67a-2, 67b-1, 67b-2) connected to load lines (4a-1, 4a-2, 4b-1, 4b-2) of said respective actuators (5a, 5b), restrictors (68a, 68b) arranged in discharge lines from said respective overload relief valves to a tank, and pilot lines (69a, 69b) connected to downstream sides of said respective overload relief valves to apply drive forces in closing directions to said respective correcting variable restrictors (56a, 65b). A flow control system according to any one of claims 1, 2, 3 and 7, characterized in that said operating variable restrictors (44a-1, 44a-2, 44b-1, 44b-2; 61a-1, 61a-2, 61b-1, 61b-2) are arranged in flow passages of directional control valves (41a, 41b; 60a, 60b) for driving said respective actuators (5a, 5b) in both normal and reversed directions. A flow control system according to any one of claims 1 to 3, characterized in that said actuators (5a, 5b) are actuators of a hydraulic work vehicle and said operating variable restrictors are operated by a control lever of said hydraulic work vehicle.

Inventors: KATO HIDEYO, OCHIAI MASAMI, KATO, HIDEYO, OCHIAI, MASAMI
Applicants: HITACHI CONSTRUCTION MACHINERY, HITACHI CONSTRUCTION MACHINERY CO., LTD.
Application year: 1994
Main classification: F15B11
IPC codes: E02F9, F15B11